Instrument for measuring small visual angles



L. K. BELL.

Patented Ian. I7,v |899.

' INSTRUMENT FUR MEASURINE SMALL VISUAL ANGLES.

(Application led Dec. 13, 1897.)

r'No Model.)

4 Sheets-Sheet I.

No. 6I7,895. Patented Jan. |7 |899.

L K. BELL.

INSTRUMENT FOR MEASURING SMALL VISUAL ANGLES.

(Application iled Dec. 18, 1897.)

@No Model.) 4 Sheets-Sheet 2.

Fig. 1'"l Fig. 1 b

Patented lan. I7, |899. y L. K. BELL.

INSTRUMENT FOR MEASURING SMALL VISUAL ANGLES.

`(Applicatiun filed Dec. 13, 1897.)

4 Sheets-Sheet 3.

@No Model.)

Patentedlan. I7, |899.

4 Sheets-Sheet 4.

un mm1 n sa u f?) um L. K. BELL.

INSTRUMENT FUR MEASURING SMALL VISUAL ANGLES.

(Application led Dec. 13, 1897.)

@No Model.)

NIT-ED STATES LEWISIQBELL, on THE BRITISH ROYALI NAVY. i

nTsN FIC@ INSTRUMETFOR MEASQRING SMALL VISUAL ANGLES.

srEcIFIcA'rIoNl form'ing'part of :Letters Patent 116.617,895, dateacanaary 17, 1899.

l A. Application iil-ed December 1S, 1897. Serial No. 661,600. (N o model.)

To all whom it may concern: Beit known that I, LEWIS KNOX BELL, a subject of the Queen Great Britain and Ireland, a commander in the Royal Navy, at present 0,11 the Mediterranean Station, have invented a new and Improved Instrument for Measuring Small Visual Angles and for the v Determination of Distances Thereby, (for which I have obtained a patent in Great' Brit? ain, No. 7 ,97%, dated April 15, 1896,) of which the following isa specification.

My invention has for its principal object toy provide'a simple and eiiicient instrument for the measurement of small visual angles in such a manner as to insure an exaggerated reading on an arc or scale, and-is Aprimarily intended for'determining the distance of objects atsea or on land by observing from a known height the angle subtended by th/e :horizon and the water-line of an object at sea, or the angle of elevation of'or the angle subtended by an object of known height or dimensions, or -by observing the depressionangle, or any other well-known method of determining distance by the measurement of visual angles, a graduated arc or scale being provided to enable'the distance corresponding to the angle observed to be read E directly, vas well asjthe angle measured. For this purpose 'I employ a prism of known angle of minimum deviation so mounted that itgcan be rotated inthe plane which bisects its re-l fracting-angle perpendicular to the lineof sight through an angle of ninety degrees. If a point or object on the horizon be now viewed, the prism-holder may be so held that the refracted image in the prism will appear horizontally apart from the directly-'viewed image and at an 'an gular distance from it equal ,to the angle of minimum deviation ofthe prism. If lthe prism be then rotated in the same plane by turning the holder thereof through an angle of ninety degrees, the refracted image will move on the circumference of a circle describedA about the point or object until it appearsvertically above or belowy v thelairjs vof the 4prism is verticale-the h'izo'n l*the directly-viewedimage, and the portion of the horizon seen on either sideof the point or objectwill undergo displacement in a similar mann er.v Whenthe 'refraction takes place only iin; a horizontal" directio-tliat is, gvhen as seen through the prism and by direct vision the holder of the prism is rotated through an angle of ninety degrees the directly-viewed and refracted images of the horizon will be separated vertically to an angular distance equal to the angle of minimum deviation of the prism. Ifthe rotatory movement ofthe prism be arrested at any point between these positions of only horizontal and only vertical displacement, thevertical angle subtended by the directly-viewed and refracted images will be equal to the angle of deviation of the will appear in an unbroken line, and when prisni multiplied by the line of the angle rof tated. For. instance, if the minimum angle of deviation of the prismv be sixtyminutes then if the holder of the prism be rotated in the manner described' through an angle of thirty degrees from the position of only horizontal displacement the vertical angle subtended by the directly-viewed and refracted images of the horizon will be sixty minutes multiplied by thirty degrees-that is, thirty' minutes. This rotatable prism is hereinafter referred to as prism A. Also, if a prism be ixedly attached to a support or frame the latter may be so placed or held in the' hand that the directly-viewed and refracted images of the horizon are seen in an unbroken linethat is, when it is. so held that refraction in the prism only takes place in a horizontal direction, and if this position be lost it may be recovered by moving the support in the same plane-?. e.,.the plane of the prisms axis perpendicular to the li'ne of sightf--until continuity of the horizon is regained., This fixed prism is hereinafter referred t0 as prism B.

' For useat sea in measuring small visual angles for the determination of distance the instrument may consist of two principal parts capable of being rotated on the same -axis one about the other through an angle of ninety degrees. One part forms the support or frame of the instrument and may be a disk of metal 'or other suitable material having am axial hole or aperture in or about which a tube can be rotated. The other part consistsof the said tube having attached to'it circumferentially4 a disk of a diameter smaller than that of thesupport or frame with which it is in contact, the ring of the latter surrounding its circumferencebeing used for graduating the IOO.

instrument, and the common axis of tube and support is the line of collimation f the instrument.

In the tube I Iix the prism A, whose minimum angle of deviation is approximately' equal to and not less than the largest angle which the instrument is required to measure. This prism may be of semicircular form and `is fixed in the position of minimum deviation in a plane perpendicular to the axis of the tube, the base of the prism being preferably toward the axis of the tube.. s

In the hole or aperture in the support I fix the prism B, which is of fairly large refracting-angle (about three or four degrees,) also in a piane perpendicular to the axis of the tube, this prism being preferably arranged Withits summit toward the axis of the tube and so that when the horizon is viewed through the tube a portion of it will be seen by direct vision between the edges of the two prisms, and also onthe one hand through the prism B and on the other hand through the, prism A. The support 'can thus be held in such position that the horizon as seen by direct vision andthrough the prism B is an unbroken line, `and maintaining this position of the support the tube can be rotated until the horizon as seen inthe prism A is continuous in the same line. Both prisms are then refracting only in a truly horizontal direction, and this is the observing position of the support and the zeroposition of the tube, which can be indicated by an indicating-point, as ct', Fig. 1, on the disk attached' to it coinciding with the initial point of graduation on that part or ring of the support surrounding it. If the support be maintained in this position, the tube can be rotated until the horizon as seen directlyand through the prism B touches the water line, viewed through the prism A, of an object at sea whose distance it is desired to determine, and by the angle of rotation of the tube from the zero position, as described, the vertical angle subtended between the direct and retracted horizons is obtained, and consequently .the angle between the horizon and the water-line of the object from which anglethe distance can be determined, the height of the 'eye of the observer above the sea being known. In a similar manner angles of elevation above the horizon-such as thealtitude of distant land, ships mastheads, and the like-can be measured.

A scale may be' made on, say, each of the four quadrants of the support, placing four indices a' b b b', the several indices b'. being radially adjustable by means of the set-screws c, t o which they are attached and which are movable in the radial slots s in the disk d on the movable circle, so that the instrument may be graduated on one quadrant to read direct the vertical angle measured, on another quadrant the distance of an object due to the angle measured at certain heights, on

another quadrant :the distance at certain other heights, and so on, or the graduations may be so made as to give the distance for all practical heights either of the observers eye or of the object whose elevation is observed. I

A telescope may be attached to the tube, and the prism A may be placed in the inanner described either before or behind the object-glass, or it maybe attached to the said glass.

Figure 1 of the accompanying drawings represents in elevation and., Fig. 2in horizontalsection an instrument constituted according to this invention. Figs. l and 1b are diagrams showing diierent positions of the prisms. Fig. 2a is ai diagram illustrating the manner of constructing the scale. Fig. 3 is a front elevation of a modified form of in'- strument. Figs. 4 and" F represent in front elevation and vertical section, respectively, another modification in the form of the instrumentland means for rotating the prism; and Figs. 3, 3", and 3c are diagrammatic view illustrating a modification.

.Referring to ,Fi/gs. l and or holder of the instrument, of circular form, provided with a handle A and carrying on one side a tube D, having an opening in its side through which projects to within a short distance of the axial line of thel tube a prism B, the said 4prism being xed in a case D2, connected by screwsto the body or holder C. On the side of the body or holder C opposite that which'carries the tube D is fitted so as to be capable of rotation a tube E, carrying another prism A, having a less angle than the prism B. The laxes of the tubes D and E are in line one with the other, and the tube E is shown as being connected to the bod-y or holder C by screws w, 4passing through a flange d on the tube and through slots lw 1n the body or holder concentric to the axes of the tubes E and D.

The prisms A andB when in their normal 2, c is the body" IOC position have their edges a b parallel one to refraction taking place to the right or left;

but if the holder C be turned to the right or to the left the horizon will appear to be twice broken, as shown, for instance, in the diagram Fig.` 1b, the prism B of larger angle throwing the portion v of .the horizon seen therethrough considerably farther out of the line than doe's the prism A of smaller angle. Now if the instrument be held so that the horizon appears asshown indiagram Fig. l and then the tub/e E, carrying the prism A, be turned axially a portion of the horizon will be refracted in the-prism A and subtend an angle, with the horizon seen through the prism B land directly through the space bei ample, forty-eight minutes. divisions draw lines parallelto the linea: y

. or index-mark a.' to indicate on the scale marked on the holder C the angle through which the tube E and pris'm A are turned.

Each quadrant of the circular body or holder 5 C (the circle being struck from the axisof the tube E) maybe graduated-viz., one

" quadrant S forangles, one, T, for distances according to the` height in feet of the observers eye when observing from the bridge of ro the vessel, one, U, when'observing from the conning-tower, and the other, V, for observing from some other height, so that the same instrument can be used for ascertaining distances at whatever height the observer :5 maybe. .if A

The following is a simple method of constructing the scales:` Let Fig. 2, represent the axis of the tube E, which carries the prism A; a: y, radius of arc y .au to be-graduated.

2o Erect a perpendicular .fr u on a: and divide the line a; u into the same number of equal parts as the angle of minimum deviation of the prism A contains minutes-say, for eX- Through these to the arc y z u. Then when the tube E,'car rying the prism A, is rotated the mark on the are where the pointer on the ange of the tube reaches indicates on the are the number 3o of minutes measured by the'prism A. For example, suppose the line a: u to be divided into forty-eight divisions, corresponding to the number of minutes in the-angle of mini-1 mum deviation of the prism A, as before del 35 scribed, and the tubeE be rotated from .its

zero positionthat is, when the edges a b of `the prisms A B are parallel to each other-till the pointer a' on the langeof the tube reaches the division on the arc-for instance, 4o at z-the angle measured will be thirty-five minutes. The graduations for the measurement of angles being found, as described, the other quadrants of the circle can be graduated for measuring distances.

scale and be reproduced on a scale suitable to the'instrument. For instance, iffit be required to graduate on an arc "y* a* u* of, say, two inches radius adistance of one. thousand 5o yards, the height of the observers eye being `thirty-five feet the angle to be measured is 34.69 minutes.

draw a line parallel tow y from this point through the large arc y z u, and from-the servers eye.

for 'the dierentheights` of the observers eye.

The divisions' 45 should be made in the first instance on alarge Set off on the Vertical or` radius line of the arc 'cc u 34.69 minutes, and

\ The angles are calculated from the formula where a angle subtended between the horizon and water-line of the object the dip due to height of observers eye in minutes.

1,146 one-third radian or unit angle (57- 17- 44{-) expressed in minutes. 75

h height vofeye in feet.

d: distance in yards.

-O R curvature refraction.

At 500 yards, 0.04 feet added to height'of eye. 1,000 yards, 0.20 feet added to height of eye. 1,500 yards, 0.42 feet added ,to height of eye. 2 ,000 yards, 0.71 feet added to height of eye.

2,500 yards, 1.07 feet added to height of eye.

3,000 yards, 1.50 feet added to height of eye.

and proportionately at distances between the above..

The values of the dip used are- 20 feet, 4.30 minut/es. 25 feet, 4.87 minutes. 30 feet, 5.25 minutes.

The flange of the tube E is providedwith radiallyfadjustable pointers b', one kfor each of the three quadrants containing the scales IOO The instrument isprovided with a telescope F, as shown in Fig. 2, screwed into the I-n the apparatus illustrated by Fig. 3 only one quadrant isjprovided, the said quadrant containing scales or grad nations for indicat ing angles and also for distances. To the tube E, carrying `the prism A, is connected an arm or radius-bar E', by which the/tube and prism can be turned axially in the body or holder C'. The scales may be marked on the body or holder O itself; but'it is preferred to mark the scales on a separate piece and attach it to the holder, so that it can be removed/ d replaced by another scale when desired. T e outery edge of the quadrant is graduated for angles in minutes, and theradius of the quadrant is graduated for heights of the ob'- servers eye-say from twenty tosixty feet. The radius-bar is fitted with an adjustable :zo pointer b, which can be `slid along the radius-bar to the division on the radius-line of the quadrant-corresponding tothe height/of the observers eye and can be fixed in its adjusted position iby a bindingnut c'.

The forward edge d' of the radius-bar serves as a pointer to indicate on the graduations on the edge 'of the'quadrant the angle in minutes measured. i 'j'.f/

The radius-bar can be fixeefby means of IIO the clamping-screw and nut d' in the position to which it has been moved. For an instrument for general use at heights varying .from twenty to sixty feet the angle of deviand upper-deck batteries of battle-ships-an angle of about twenty-four minutes will be sufficient.

In the arrangement illustrated by Figs. 4 and 5 the tube or holder E of the prism A is arranged eccentrically to the center of the disk C and is provided with a toothed segment E2, with which gears a wheel H, mounted on a stud in the center of the disk C. To the wheel Il is attached a radius arm or bar E", carrying in bearings e' a screw-threaded spindle I, on which is fitted so as to traverse therealong a nut b2, provided with an indexpointer Z912 to indicate on the scale the height of the observers eye, the screw being rotated for the purpose of adjusting the index-pointer 1912 by means of a milled head I2.

The radius-bar E" is provided at its outer end with a depending lip d2, between which y lip and the periphery of the disk C is interposed a spring f forthe purpose of causing sufficient friction between the radius-bar and the disk C to retain the bar in the position to which it is adjusted. The front edge a of the radius-bar serves as the index to indicate on the scale the angle in minutes measured.

In constructing an instrument for use on land the movable prism B may be dispensed with and the prism A be fixed in front of the object-glass ofa terrestrial telescope, the eyepiece of the telescope being provided inside with cross threads or wires B', as shown' diagrammatically in Figs. 3, 3b, and 3c. The prism A is arranged so as to cover about onehalf the field-of view, and its zero position is such that'its summit is parallel to one of the cross wires or threads andf perpendicular to the other cross wire or thread. To the tube of the telescope is attached a disk C, having one quadrant thereof graduated to angles and another quadrant graduated for distances corresponding to,-. say, five and one-half feet in height, about the height of a man, and the eyepiece carries a disk or index, which works in close proximity to the disk C. Now if, for example, it be wished to obtain the angle subtended by a man standing at a distance the tube of the telescope is adjusted so that the prism measures the maximum vertical angle, and then the telescope is directed to the object (the man) and turned bodily until the direct image and the retracted image of the object are in a vertical lin'e on or parallel to vthe vertical cross-Wire, as shown in the diagram Fig. 3b, the upper figure being the direct view and the lower figure the retracted View of the object. Retaining the eyepiece of the telescope and the cross-wires'in this position, turn the tube containing the prism until the'top of the lower figure and the bottom of the top figure lsimultaneously touch the horizontal Wire or thread, as shown in Fig. 3c. The rotation thus given to the prism measured from the zero-point on the scale indicates the angle and distance, as hereinbefore described. The scales for use with this instrument are similar to those shown in Figs.

-1 and 3, the oblique lines being, as before,

lines of distance, while the radial graduations indicate heights of objects observed. The

angle a., subtended at the eye by an object at a'distance, is given by the formula:

height of object distance and in constructing the scales a table of angles (in minutes) calculated by this formula is to be used, as before-described. Before makin g an observation the adjustable pointer or index is set to the height (known or estimated) of the object whose distance is to be measured. The same principle can be -applied to the construction of similar instruments on a binocular form.

Having now particularly described and as- Tan. a z:

certained the natu re of my said invention and.

in what manner the same is to be performed, I'declare that what I claim is;-

1. In an instrument for measuring visual angleslthe combination with a prism of known angle of minimum deviation rotatable in a plane perpendicular to the line ofl sight and occupying but a portion of the field of vision, of a fixed refracting-prisln occupying a different portion of the field of vision, by means of which the instrument can be adjusted to the correct position for making correctobservations, substantially as described.

2. In an instrument for measuring visual angles and distances the combination with a prism of known angle of minimum deviation, rotatable in a plane perpendicular to the line of sight and occupying but a portion of the iielrd of vision, of a ixed refracting-prism occupying a different portion of the field of vision, by means of which the instrument can be adjusted to the correct position for making- IOO IIO 

